JavaScript · 40 MCQs with explanations
JavaScript runs in every web browser and, through Node.js, on servers. It is forgiving, which makes it easy to start with but also full of surprises. Interview questions tend to target exactly those surprises, so these notes focus on them.
== converts types before comparing ("1" == 1 is true); === does not. Use === by default. The + operator joins strings if either side is a string, while -, * and / convert to numbers.var is function-scoped and hoisted with the value undefined. let and const are block-scoped and cannot be used before their declaration (the “temporal dead zone”).| Expression | Result | Why |
|---|---|---|
"5" + 3 |
“53” | + with a string joins text |
"5" - 3 |
2 | – converts to numbers |
0 || "x" |
“x” | || skips any falsy value |
0 ?? "x" |
0 | ?? skips only null and undefined |
1. What does typeof null return?
Answer: C – “object”. This is a long-standing bug from the first version of JavaScript, kept for compatibility. Check for null with value === null.
2. What is the difference between == and ===?
=== compares value and type without conversion; == converts types first== is faster and stricter=== only works with numbersAnswer: A. 0 == "" is true but 0 === "" is false. Strict equality avoids most comparison bugs.
3. How is let different from var?
Answer: B. A let declared inside an if or loop exists only inside that block. A var leaks out to the whole function.
4. const user = {name: "Ali"}; user.name = "Sara"; What happens?
Answer: D. user = {} would fail. To make an object’s properties read-only, use Object.freeze(user).
5. What is "5" + 3?
Answer: A – “53”. When either operand of + is a string, JavaScript converts the other one to a string and joins them.
6. What is "5" - 3?
Answer: C – 2. The minus operator only works on numbers, so the string is converted. This asymmetry with + is a favourite interview question.
7. What does 0.1 + 0.2 === 0.3 return?
Answer: B – false. Numbers are binary floating point, so the sum is 0.30000000000000004. For money, work in whole units (paisa or cents) or round the result.
8. How do you reliably check whether a value is NaN?
value === NaNvalue == NaNtypeof value === "NaN"Number.isNaN(value)Answer: D. NaN is the only value not equal to itself, so NaN === NaN is false. typeof NaN is “number”.
9. What does console.log(x); var x = 5; print?
Answer: A – undefined. The var declaration is hoisted to the top of the scope, but the assignment stays where it is.
10. What does console.log(y); let y = 5; do?
Answer: C. let and const are hoisted but stay in the temporal dead zone until the declaration line runs, so accessing them earlier throws an error.
11. What is a key difference between arrow functions and regular functions?
this; they use the surrounding oneAnswer: B. This makes them ideal for callbacks inside methods, but unsuitable as object methods that need this to be the object.
12. What is a closure?
Answer: D. Example: function counter(){ let n = 0; return () => ++n; } – each returned function keeps its own private n.
13. What does [1, 2, 3].map(n => n * 2) return?
Answer: A. map returns a new array of the same length and does not change the original array.
14. Which array method returns only the items that pass a test?
Answer: C – filter. find returns only the first matching item (or undefined). forEach returns nothing.
15. What does [1, 2, 3].reduce((sum, n) => sum + n, 0) return?
Answer: B – 6. reduce combines all items into one value; the 0 is the starting value of sum. Always pass a starting value to avoid errors on empty arrays.
16. Given const a = [1, 2], what is [...a, 3]?
Answer: D. The spread operator expands the array’s items. It is also a quick way to copy an array (shallowly).
17. What does const { city } = { name: "Ali", city: "Faisalabad" }; do?
Answer: A. Object destructuring pulls properties into variables by name. You can set defaults: const { city = "Lahore" } = obj;
18. What does user?.address?.city return if user.address is undefined?
Answer: C – undefined. Optional chaining stops at the first null or undefined instead of throwing “Cannot read properties of undefined”.
19. What is 0 ?? 5?
Answer: B – 0. The nullish coalescing operator only falls back for null or undefined, so a real 0 is kept – useful for quantities and prices.
20. What is 0 || 5?
Answer: D – 5. || returns the first truthy value, and 0 is falsy. Compare with ?? in the previous question.
21. Which are the three states of a Promise?
Answer: A. Once fulfilled or rejected (together called “settled”), a promise never changes state again.
22. What does an async function always return?
Answer: C. If you return 5, callers receive a promise that resolves to 5. Thrown errors become rejected promises.
23. Where can you use await?
Answer: B. Wrap awaits in try/catch to handle rejected promises.
24. In what order does this print? console.log("A"); setTimeout(() => console.log("B"), 0); Promise.resolve().then(() => console.log("C")); console.log("D");
Answer: D – A D C B. Synchronous code runs first (A, D). Then the microtask queue (promise callback C) runs before the task queue (timer B), even with a 0 ms delay.
25. const res = await fetch(url); How do you get the JSON body?
const data = await res.json();const data = res.body;const data = JSON(res);const data = res.json;Answer: A. res.json() also returns a promise. Note that fetch does not reject on HTTP errors like 404 – check res.ok.
26. What is event bubbling?
Answer: C. A click on a button also triggers click listeners on its parent div, body and document, unless event.stopPropagation() is called.
27. What does event.preventDefault() do on a form’s submit event?
Answer: B. This is how single-page forms validate and send data with fetch instead of a full page reload.
28. What does document.querySelector(".item") return?
Answer: D. Use querySelectorAll for all matches; it returns a NodeList.
29. Why prefer addEventListener over assigning element.onclick?
Answer: A. Assigning onclick again replaces the previous handler. addEventListener also supports options such as { once: true }.
30. What is event delegation?
Answer: C. It works for elements added later and uses less memory than one listener per row in a long list.
31. Which statement about localStorage is true?
Answer: B. Store objects with JSON.stringify and read them with JSON.parse. Any script on the page can read it, so never store secrets there.
32. What does JSON.stringify({a: 1}) return?
Answer: D. It converts values into JSON text for sending or storing. Functions and undefined properties are left out.
33. How do you reliably check whether a value is an array?
Array.isArray(value)typeof value === "array"value.isArray()value instanceof ObjectAnswer: A. typeof [] returns “object”, so typeof cannot tell arrays apart from other objects.
34. What does typeof function(){} return?
Answer: C. Functions are objects in JavaScript, but typeof reports them as “function”.
35. What does "use strict"; do?
Answer: B. ES modules and class bodies are strict automatically.
36. Which syntax creates a template literal that can include ${variables}?
Answer: D. `Total: ${qty * price}`. Template literals can also span several lines.
37. What are JavaScript classes underneath?
Answer: A. Methods defined in a class live on its prototype, shared by all instances.
38. In strict mode, what is this inside a regular function called on its own, like show()?
Answer: C – undefined. In non-strict scripts it falls back to the global object. this depends on how a function is called, not where it is written.
39. How do you load an ES module in a web page?
<script module><script type="module" src="app.js"><import src="app.js">require("app.js") in HTMLAnswer: B. Modules use export and import, run in strict mode and are deferred by default.
40. What is debouncing, for example on a search box?
Answer: D. It avoids sending a server request on every key press. Throttling, by contrast, runs a function at most once per time interval.
Questions 5–10, 19–20 and 24 cover the parts of JavaScript that surprise even experienced developers. If you got them right, you understand the language well. Next, try the HTML & CSS MCQs to round out front-end skills, or the Python MCQs. All topics are on the Programming MCQs hub.
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